Various masks are used to provide fresh air or oxygen to the airways of humans. A specialised category of masks is used to provide positive pressure to the human airway. Positive pressure applied in this manner has two different goals.
In a first category, positive pressure is applied to the lungs for the purpose of stabilising the lungs, and in particular for maintaining a minimum inflation level of the small air spaces in which gas transfer occurs (the alveoli). This therapy is very useful in patients with a variety of lung diseases, where the disease processes tend to lead to collapse (closure of the airway containing regions of the lung).
In a second category, the positive pressure is applied to the nasal airway with the intention of maintaining the pressure in, and the patency of, the upper airway. This form of positive airway pressure is known as nasal continuous positive airway pressure (nasal CPAP). This is now the “gold standard” treatment for the condition known as obstructive sleep apnea (OSA), and also for snoring and a variant of this therapy, bi-level positive pressure, is used to both stabilise the upper airway and provide additional positive pressure to support breathing. Obstructive sleep apnea is a condition in which the upper airway closes in sleep, and does so repeatedly. Nasal CPAP, when applied for the duration of sleep, stabilises the upper airway and allows for normal sleep and normal breathing.
The use of nasal continuous positive airway pressure to treat upper airway obstruction in sleep has been the subject of patents, and has been referred to in a variety of medical publications and was developed primarily for adult use. In recent years nasal CPAP has been used in the treatment of infantile obstructive apnea. However, a major problem with effectively treating an infant subject with CPAP is obtaining a mask that is appropriate for that infant. There are two issues which are critical in the effective delivery of CPAP. First, the mask must be able to maintain a known pressure in the airways during both the inspiratory and the expiratory cycle. To do so requires a hermetic seal between the mask proper and the subject's skin. Secondly, it is necessary, or at least highly desirable, to minimise and eliminate torsional movement causing twisting of the mask and consequently the leaking that arises either from movement of the subject's head, or movement of the air delivery pipe which must be attached to the mask.
The extent to which these two issues affect the use of the mask in adults as opposed to infants varies considerably. First, in the case of achieving a good seal, the adult is able to readily adjust their mask if they are experiencing a less than perfect seal (which usually results in a leak of air from the mask). In contrast, infants are unable to manage the required readjustment themselves. Secondly, problems arising from the torsional effects between the two interfaces of the air delivery pipe to the mask manifold, and the mask seal to the face are greatly exaggerated with infants as compared with adults. It is well known that a typical infant makes many more movements, particularly head movements, during sleep compared with a typical adult. In particular, the infant has many twitches and startles with concomitant head movements during dreaming sleep (rapid-eye movement sleep, known as REM). Further, the infant spends much more time in REM sleep (up to 50% of total sleep time) than the adult (less than 25% of total sleep time). Clearly the greater the number of movements, the more likely it is that the torsional effect will cause the mask to lift from the face which will lead to air leaking. Again, the adult is better able to adjust their mask to overcome this problem, than the infant.
Notwithstanding these issues, until now, infant masks have been developed on the basis of scaling down the adult mask to approximate to the infant face and nose. The problems with this scaling down process are fourfold.
First, the adult nose and middle third of the face is very different in shape from that of the infant. The adult nose is more elongated than, and protrudes far more from the surface of the face compared to the infant nose which is relatively flat, with no bridge, with the nares (nostril passages) pointing outwards. Therefore in order to fit the adult nose the base of the mask has a triangular shape elongated in the vertical axis and a “notch” region to accommodate the nose bridge. In contrast, with an infant, the width at the base of the nose approximates the height from the base of the nose (nares) to the apex of the nose (nasion). The “proportional shape” of the nasal area of an adult is rectangular and “V” shaped compared with a square and flat “proportional” shape for an infant. In addition to this basic difference in proportional shape, the adult face has quite marked contours especially around the nose and cheek area which are absent in the infant. The adult mask must therefore have acute angles which accommodate these facial contours. Thus, when an adult mask is scaled down for an infant, not only are the proportions wrong for the infant nose and face, but the angles which are unnecessarily incorporated, inadvertently introduce a new problem. Because the infant has a relatively flat nose, and virtually no bridge, the angles promote formation of channels in the sealing margin of the mask, especially in the region of the nasal bridge.
Secondly, in adult mask designs, the straps of the head harness connect with lugs on the rigid manifold of the mask in the order of 20 mm away from the surface of the face to allow the mask to accommodate the height of the adult nose. Because of this, a potential fulcrum effect is created. In the adult this fulcrum effect is not as problematic as in the infant, not only because the adult is less mobile during sleep as discussed above, but also because the contours of the adult face and cheeks can offset this rise. In the infant, when the mask used is merely a scaled down adult mask, the elevation of the strap lugs above the face is about 12 mm. This by itself creates a potential fulcrum as it does in the adult, but the effect is enhanced by the fact that there is no offset from the infant cheek due to the smaller facial area. Consequently, the straps holding the mask in place come into greater contact with the side of the face in the infant, compared to the cheek in the adult.
Thirdly, because the attachment of the paediatric mask to the face and head mimics that of the adult mask, the torsional forces are increased. The greater torsional effect is due to the decreased surface area of the mask face contact relative to the size of the air delivery pipe. Thus relatively minor movements can result in sufficient torsional forces to cause movement at the interface between the mask and the infant's face.
Fourthly, adult masks typically comprise a rigid manifold made of a hard plastic material. Straps attach to the manifold for locating the mask in position on an adult patient. The rigid manifold supports a flexible face engaging portion/seal which seals against the adults face. However such designs even if scaled down, are not suitable for infants, who may be restless during sleep. Even relatively minor movements may cause discomfort or a breakdown in sealing due to the rigid manifold contacting the infant's bed and levering the face engaging portion away from the infant's face. More serious discomfort would occur if the infant were to turn over and lie face down in which case the rigid shell would be pushed into the infant's face.
There are numerous published patents and patent applications which relate to masks for use in CPAP and for other gas supply applications. They include U.S. Pat. No. 5,243,971 (Sullivan et al), AU 42476/99 (ResMed Ltd), WO 98/18514 (Sleepnet Corp), U.S. Pat. No. 5,657,752 (Landis et al) and U.S. Pat. No. 5,650,354 (Berthon-Jones et al) which describes a combined mouth and nasal mask. These all rely on a rigid manifold and a flexible face engaging portion/seal.
All of those publications are directed to masks for adults and it is significant to note that despite nasal CPAP having been in use on infants for over twenty years no satisfactory infant mask has yet been proposed. For this reason many clinicians have preferred to use nasal prongs for undertaking nasal CPAP in infants. In spite of nasal prongs often being uncomfortable for infants, this preference has been due to the prongs' ability to provide an adequate gas seal together with the lack of an effective alternative nasal interface product.
It is an object of the present invention to alleviate some or all of the above mentioned problems with the prior art and provide an improved mask which is particularly suited for infant facial structures. For the purposes of this specification the term infant or infant human includes premature/neonatal babies, newborn babies, infants and small children having infant facial profiles who may be older than one year, possibly aged up to eighteen months to about two years old. The shape of the infant's face is the significant factor, not the age of the child.